This invention relates to hollow fibers which can be employed as reinforcing and density altering additives for polymer systems. In particular, the invention involves hollow fibers formed by spray drying solutions of film-forming materials.
The use of various fibers, especially glass fibers, for reinforcing various polymer systems is very well known. Examples of an apparatus and method for preparing such fibers are found in U.S. Pat. No. 3,265,483. While such solid fibers are suitable for reinforcing various polymer systems, they are not effective in altering the density of such materials because of their own relatively high density. Numerous other materials are used as fillers for polymers and to control the density. Among these materials are hollow microspheres such as those described in U.S. Pat. No. 2,797,201, among many others. These materials provide no reinforcing effect.
The problem of reinforcing and altering the density of polymer systems has been recognized by others. U.S. Pat. No. 4,039,718 describes hollow glass filaments sealed at both ends. These materials are difficult and expensive to prepare, involving a separate sealing step after forming the open hollow fiber. U.S. Pat. No. 3,692,507 describes silicate fibers that are formed from attenuating flows of molten silicate glass, hydrating the fibers so produced and foaming the fiber by applying heat. These methods involve costly and relatively complex processing steps, and the materials have not been adapted for fabricating reinforced plastics.
It is an object of this invention to provide a hollow reinforcing fiber by a simplified production method.